The present invention relates to an optical modulator which modulates the output light from a light emitting source, and more particularly to a semiconductor external optical modulator which is adapted for improving the spectral width of light during high-speed modulation.
By reason of its small size, high efficiency and high reliability, a semiconductor laser has already been put to practical use as the light source for optical fiber communication. Another note-worthy feature of the semiconductor laser is to permit direct modulation, but high-speed direct modulation causes an increase in the spectral width of the semiconductor laser output light, constituting a serious obstacle to long-distance, large-capacity optical fiber communication. An ordinary semiconductor laser which utilizes the cleaved facets oscillates at a plurality of wavelengths during high-speed modulation, and hence it is employed only in the 1.3 .mu.m wavelength band in which the output light is free from wavelength dispersion by the optical fiber but suffers a great loss. On the other hand, since a distributed feedback semiconductor laser, which operates at a single wavelength even during high-speed modulation, is insusceptible to the influence of the wavelength dispersion, it is now being developed intensively for use in the 1.5 .mu.m band in which the loss of the output light is small. It has been clarified, however, that with a modulation rate above 1 Gb/s, even if the laser oscillates at a single wavelength, the spectral width of the output light increases owing to its frequency modulation by the varying injected carrier density, resulting in the problem that the influence of dispersion is non-negligible.
Accordingly, chirping of the oscillation wavelength or the resulting increase in the spectral width during high-speed modulation is unavoidable as long as the semiconductor laser is subjected to direct modulation. In this sense, the external modulation technique is considered promising which allows the semiconductor laser to operate at a single wavelength in the steady state and modulates the output light outside the oscillator. With the external modulation technique, in an ideal case where the static spectral width (.ltoreq.10 MHz) increase by the width of the modulation band (.about.GHz) alone, the increase in the spectral width can be reduced down to about 1/10 that (1 to 3 .ANG.) in the case of the direct modulation.
As such a conventional waveguide type external optical modulation element, directional coupler type and Mach-Zehner interferometer type structures ulitizing a ferroelectric material are mainly attracting attention in terms of the modulation band and the extinction ratio. However, these modulators are defective in that they cannot be integrated with the semiconductor laser, that since they perform intensity modulation of light by changing its phase velocity, their fabrication calls for uniform and precise control of dimensions of the waveguide, and that since the amount of variation in the phase velocity per unit length is small, the device length is as long as several millimeters to several centimeters so as to obtain a required amount of variation in the phase parameter, resulting in a great insertion loss.
To avoid such defects, there has been proposed an electroabsorption type optical modulator which employs a semiconductor material so that, through utilization of an electroabsorption effect unobtainable with the ferroelectric material, an electric field is applied by an external voltage to the optical waveguide layer to change its absorption coefficient, thereby modulating the intensity of light. This optical modulator can be driven at low voltages and is small in length, easy to fabricate and high-speed, and is now attracting attention as an optical modulation element which can be integrated with the laser. It has recently been pointed out, however, that in the absorption type modulator the absorption coefficient and refractive index of its optical waveguide layer both undergo variations with the applied electric field and the intensity modulation is accompanied by a phase modulation, with the result that during the high-speed modulation the spectral width will increase as in the case with the direct modulation of the laser (Koyama and Iga, Electronics Letters, Vol. 21, pp. 1065-1066, No. 1985).
As described above, the conventional semiconductor external optical modulator inevitably increases the spectral width of its output light, and hence has the shortcoming that it cannot be employed in a large-capacity, high-speed optical communication system.